This invention relates to a method and compound for inhibiting the plugging by gas hydrates of conduits containing a mixture of low-boiling hydrocarbons and water. In particular, it relates to a method and compound for continuing the flow of a mixture of hydrocarbons and water through a conduit after a shutdown which caused separation of hydrate crystals from the mixture.
The problem of the formation of gas hydrates (clathrates of gases in a lattice consisting of water molecules) is well known in the art. Low-boiling hydrocarbons, in particular methane, may under conditions of elevated pressure and reduced temperature tend to form gas hydrate crystals with water present in natural gas or crude oil.
Such gas hydrate crystals, when allowed to form and grow inside a conduit such as a pipeline, tend to block or even damage the conduit. A number of methods has been suggested to prevent such blocking of which the use of crystal growth inhibitors is considered to be very attractive.
In International Patent Application Publication WO 96/34177 the use of a class of hydrate formation inhibitors is described having the general formula (R.sub.1) (R.sub.2) (R.sub.3) (R.sub.4)A.sup.+ Y.sup.- wherein two of R.sub.1 -R.sub.4 are independently normal or branched alkyls having 4 or 5 carbon atoms, two of R.sub.1 -R.sub.4 are independently representing organic moieties having at least eight carbon atoms, A represents a nitrogen or a phosphorus atom and Y represents an anion.
Attractive results have been obtained when using the di-ester of di-butyl-di-ethanol ammonium bromide and coconut fatty acid, both with respect to decreasing the temperature at which hydrates tend to be formed (as described in experiment A 3b of said patent specification) and with respect to restarting the flow of the medium after stopping (as described in experiment B 1b of said specification). In the experiment B 1b it is described that after a temperature decrease of 11.degree. C., followed by stopping the circulation of the medium used which resulted in the slow separation of a layer of very fine hydrate crystals from the hydrate forming medium, the circulation could be restarted whereby the layer of loose powder hydrates became readily resuspended into the hydrocarbon liquids resulting in the formation of the hydrate suspension which was observed prior to shutdown.
One of the problems still to be solved in managing the flow of suspended gas hydrates in conduits is being able to restart the flow after an interruption, either scheduled or unscheduled, when powder hydrates have been formed under rather severe conditions. Even with the preferred di-ester of di-butyl-di-ethanol ammonium bromide and coconut fatty acid restarts at rather severe conditions appear to be problematic.